From: dpierce@cartchunk.org   
      
   Don Pearce wrote:   
   >>Apart from EMI, is there a reason?   
   >   
   > Yes. That high frequency square wave would dissipate maximum power   
   > into any tweeter connected to it - even without any audio signal   
   > present. It has to be suppressed.   
      
   Well, at the kind of frequency we're talking about,   
   in the undreds of kilohertz, both the series inductive   
   AND series resistance component of any dynamic tweeter's   
   impedance is substantially higher than in the audio range,   
   and is increasing with increasing frequency, The result   
   is you end up having a substantially higher load impedance,   
   thus have less power dissipation, in series with an inductive   
   filter, thus have less power to dissipate.   
      
   The issue for me is that there are some examples of speakers   
   whose impedance in the hundreds of kilohertz regions are not   
   so well defined. FOr example, be wary of ANYTHING with a piezo-   
   electric driver in it, because they can hit a nasty resonance   
   wuite a ways up, where the impedance is at a sharp minimum   
   and then you have series of irregularily space impedance   
   minima and maxima dues to all sorts of nasty modes.   
      
   But, I agree, the EMI issue is VERY significant.   
      
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